In general, the present invention relates to a method and apparatus for shimming a magnet. In particular, the present invention is a method for determining the placement of shim elements, and an apparatus used to place the shim elements in specific locations on the pole surface of a magnet in order to shape the magnetic field to achieve greater uniformity.
Magnetic resonance imaging (xe2x80x9cMRIxe2x80x9d) is one of the most versatile and fastest growing modalities in medical imaging. As part of the MRI process, the subject patient is placed in an external magnetic field. This field is created by a magnet assembly, which may be closed or open. Open magnet assemblies have two spaced-apart magnet poles separated by a gap, and a working magnetic field volume located within the gap.
The diagnostic quality of images produced by MRI is directly related to several system performance characteristics. One very important consideration is the uniformity, or homogeneity, of the main magnetic field. In order to produce high-resolution images, the magnetic field produced in the MRI scanner must be maintained to a very high degree of uniformity. However, an MRI magnet initially produces a field that is usually less uniform than that required to image successfully. At some point after manufacture, the magnet assembly must be adjusted to produce a more uniform field.
A process known as shimming is used to improve the homogeneity of the magnetic field to the necessary levels by making small mechanical and/or electrical adjustments to the overall field. Mechanical adjustments are called passive shimming, while electrical adjustments are known as active shimming. Electrical adjustments are effective because electrical current passing through a wire will produce a magnetic field around that wire. When these wires are formed into coils, the strength, direction, and shape of the magnetic field produced can be controlled by adjusting the physical and electrical parameters of the coils. Placing these coils in strategic locations as secondary magnetic field sources has the effect of adding to or subtracting from the main magnetic field in localized regions as well as over the entire pole surface, affecting the overall homogeneity of the main field. While the use of these xe2x80x9cshim coilsxe2x80x9d has allowed the homogeneity of the main MRI magnetic field to be greatly improved, there are numerous drawbacks associated with their use.
For example, the electric current in the shim coils may be unstable, resulting in an overall instability in the main magnetic field. This instability may cause xe2x80x9cghostingxe2x80x9d in the MR images. Ghosting is an interference phenomenon that appears at periodic intervals along the phase axis. These errors are unacceptable to any radiologist, who may confuse the correct position of the patient""s anatomic elements, possibly resulting in an incorrect diagnosis.
Further, shim coils are temperature sensitive. Variations in the temperature of the individual coils can cause instabilities in the main magnetic field, resulting in image artifacts. In addition, the currents used to produce the magnetic fields in the shim coils require complicated electronic circuits, such as voltage and current regulators and current amplifiers, to maintain stability. The shim coil can become inoperable when one or more of these electronic components breaks or goes out of tolerance. Even when all the electronic components are working properly, this type of active shimming adds expense and complexity to the overall MRI system. Passive shimming avoids adding complexity to the MRI system, but instead makes manufacture of the magnets more complicated, usually by requiring the custom physical modification of the magnet core components, such as shim bars, while adjusting the uniformity of the field produced by the newly-manufactured magnet.
To overcome these and other shortcomings of the active (coil) shimming process, the present invention eliminates or minimizes the use of some or all shim coils and their associated currents altogether, achieving a high degree of field uniformity required for high resolution imaging through a process using only passive shimming. The shimming is effected through the use of metal shim elements which are added to the standard magnet in order to physically influence the overall field produced by the magnet. The shim elements may be held in place by a non-metallic cover which is affixed to the magnet. Preferably, a metal plate may be applied between the magnet and the non-metallic cover to incorporate a combination of shim elements in the form of additive metal elements, and other shim elements in the form of areas where metal has been removed from the metal plate.
According to a first aspect of the present invention, a method for passively shimming a magnet includes measuring a magnetic field produced within a predetermined volume by the magnet, modeling the measured magnetic field in the form of a polynomial expansion having a predetermined number of harmonic terms, detecting the degree of homogeneity in the measured field within the predetermined volume by examining the harmonic terms, determining which harmonic term of the polynomial expansion should be modified in order to change the homogeneity detected in the measured field, and designing a metal element which would provide the determined harmonic term modification when coupled to the magnet. Magnetic field data is produced during measurement, and this data is analyzed and described as the polynomial expansion during the modeling. Preferably, the polynomial expansion is a Legendre expansion, and the metal element is a shim. The shim may be a metal ring or an arcuate metal element, and is coupled to a pole of the magnet assembly. The method may also include attaching a non-metallic cover plate to the magnet, wherein the cover plate includes grooved portions for holding the shim in place in a predetermined location with respect to the pole of the magnet assembly. The method may further include disposing a metal plate such that the metal plate affects the magnetic field produced by the magnet, and such that the shim is held in the predetermined location between the metal plate and the cover plate. The method may also include removing a portion of the magnet or the metal plate corresponding to the metal element in order to modify the determined harmonic term.
According to another aspect of the invention, an apparatus for changing a homogeneity of a magnetic field produced by a magnet includes a first plate for placement near enough to the pole of the magnet so as to have an effect on the magnetic field, a shim for placement near enough to the first plate so as to have an effect on the magnetic field, and a second plate for attachment to the first plate such that the shim is held in place in a predetermined location between the first plate and the second plate. The second plate includes means for holding the shim in the predetermined location preferably a grooved surface. Preferably, the first plate is made substantially of metal and the second plate is made of a non-metallic material. The shim is made substantially of metal, and may be a metal ring or an arcuate metal element. The shim corresponds to a magnitude change of a harmonic term of a polynomial expansion describing the magnetic field in order to change the homogeneity of the magnetic field, and the first plate may include a grooved portion which corresponds to a magnitude change of a harmonic term of a polynomial expansion describing the magnetic field in order to change the homogeneity of the magnetic field.
According to a further aspect of the present invention, an apparatus for changing a homogeneity of a magnetic field produced by a magnet includes a shim and a plate for attachment to the magnet such that the shim is held in place in a predetermined location between the magnet and the plate. The plate includes means for holding the shim in the predetermined location, preferably a grooved surface. The plate is made of a non-metallic material. The shim is made substantially of metal, and may be a metal ring or an arcuate metal element. The shim corresponds to a magnitude change of a harmonic term of a polynomial expansion describing the magnetic field in order to change the homogeneity of the magnetic field. The apparatus may include a grooved portion in the magnet which corresponds to a magnitude change of a harmonic term of a polynomial expansion describing the magnetic field in order to change the homogeneity of the magnetic field.